The grey scale of an electrophoretic display device is usually generated by applying a series of discrete pulses to the display media. The electrophoretic media, however, is not linear with the number of pulses. The optical response curve is in fact quite steep in the middle of the grey zone and less steep near off and on states. Therefore minor changes in driving time or voltage in that middle grey zone may cause a significant change in reflectance. Also, since there are a limited number of pulses between off and on, it is often difficult to obtain the desired reflectance. This can lead to undesirable non-uniformities and a poor match to a curve of target reflectance vs. input level, often referred to as a “gamma” curve.
In order to better match the gamma curve with driving characteristics of an electrophoretic display, the bandwidth of the pulse-width modulation could be increased so that there are more steps available between off and on. By having shorter pulse widths, the reflectance can be controlled more precisely. However, this approach has the disadvantage of requiring a complex hardware platform (especially for the active matrix display devices) with higher costs.